The present invention relates to improvements in keyboards and is particularly concerned with a device which prevents particular keys from being depressed as the first one (or another one) within a series of key depressions.
In certain instances it is desired, for example, that a telephone (with a push button keyboard) can be used to dial, e.g. local numbers only or can be used in-house only and not for dialing out. This result can be achieved by blocking the depression of certain keys, e.g. as first ones. For example, in Europe, long distances calls must begin always by dialing a zero as the first digit. Blocking the depression of the zero key as long as no other key has been depressed first, effectively inhibits the making of long distance calls. In some parts of the USA, long distance calls have to be preceded by dialing a "1". In other parts a long distance call can be recognized in that the second dialed digit is either a "1" or a "0". In other cases, (hotels, offices) dialing out is preceded, e.g. by dialing a 9. If such a dial out is to be prevented, e.g. from certain phones, the first dialing of a 9 must be inhibited.
Inhibiting the depressing of particular keys as first ones being depressed in a sequence of key depressions is not known up to now, but in the field of telephone interconnections it is already known for a long time to block certain lines of communication if there is dialed a zero as the first digit. This blocking is achieved by providing a counter counting the dial pulses which energizes a blocking relay blocking the connection to the calling telephone set if the number of the first dial pulses correspond to dialing a zero. This known arrangement is expensive and not safe against overcoming the blocking.